For the measurement of liquids, various detection methods such as a weight type detection method (load cells). pressure type detection method (differential pressure transmitters) and volume type detection method (oval flow meters) have been employed. For measurement of powders a weight type detection method using a load cell has been mainly employed.
In any one of the detection methods, it is an essential premise for measurement control that the flow speed is constant. A closed loop measurement control method in which the flow speed is continuously varied has not yet been proposed in the art.
In order to improve the measurement accuracy, the following methods have been employed.
In a first method, the flow speed in a flow control valve is changed in two steps so that, when the measurement target value is approached, the flow speed is switched over to a slow flow speed (c.f. Japanese Patent Application (OPI) No. 148019/1981). In a second method, an amount of inflow (or amount of head) is set as a measurement stopping condition, and the amount is predicted to suspend the measurement (c.f. Japanese Patent Application (OPI) No. 29114/1982). (The term "OPI" as used herein means an unexamined published application.)
In the conventional measurement control the flow speed is constant or it is switched in two steps as was described above. However, in a certain range, the measurement is carried out with the flow rate fixed. Therefore, the conventional measurement control suffers from the following disadvantages.
The first disadvantage is the measurement accuracy. The measurement accuracy may not be guaranteed because of the variation of flow speed which is caused by disturbance or by the variation of physical characteristic (such as viscosity) of the liquid.
For instance in the conveyance of a liquid by gravity, the residual amount of liquid to be measured remaining in the upstream container (referred to a "a head difference" in this specification) affects the flow speed of the liquid. If the head difference changes greatly, then the flow speed goes out of a certain range, thus adversely affecting the measurement accuracy. As a result, the head of the upstream container must be limited to remain in a narrow variation range. Therefore, in order to maintain the head difference in a predetermined range, it is necessary to suspend the measurement or to supply a suitable amount of raw material to the upstream container, with a secondary loss of raw material.
The second disadvantage is the measurement range. Since the flow speed is limited, the ratio of the minimum measurement value to the maximum measurement value is of the order of 1:5.
In the case where the flow speed is switched in two steps the ratio is of the order of 1:10 at maximum.
The reason why the measurement range must be narrow, as described above is as follows. Even if the measurement is suspended, some amount of liquid flows in because of the delay in response of the system. The amount of inflow is determined by the flow speed. Therefore in the case where the measurement target value is small, the amount of inflow exceeds the accuracy-guaranteed limit. As a result, the measurement range is limited.
In a plant which manufactures a variety of solutions, for one and the same raw material a measurement range of about 1:100 at maximum is available, and it is necessary to select a measuring apparatus in a range of measurement target values.
The third disadvantage is the measurement time. The measurement time depends on the measurement target value.
When the measuring target value is small, the measurement time is short: and when large, the measurement time is long.
When the measurement target value is small, the operation time of the system fluctuates and the measurement accuracy cannot be guaranteed, with the result that the measurement range is decreased.
In a system of mixing a plurality of measured solutions to prepare a new solution, the manufacturing capacity depends on the measurement time. Especially in a pipeless movement type manufacturing system, the conveying capacity is limited.
The above-described difficulties result in economical disadvantage in the formation of the production line. That is, heretofore, a number of liquid and powder measuring apparatus are installed according to measurement target values. Such a measuring apparatus is provided for each optimal measurement time determined from the limitation of manufacturing capacity, or for each raw material to be handled. That is, a large number of measuring apparatuses are installed.
Recently, one of the present applicants has developed and filed a patent application on the following technique in order to provide a liquid and powder measuring method in which the above-described difficulties are eliminated. The measurement is carried out with high accuracy being free from the variation of flow speed due to disturbances and the variations of liquid physical characteristics (such as viscosity) and of powder physical characteristics (such as fluidity). A wide range of measurement is established and a short time measurement is realized independent from the measurement target value.
That is, Japanese Patent Application No. 106412/1987 discloses a closed loop liquid measuring method in which a freely determined measurement target value and a fed back actual measurement value are utilized to change the flow speed. In this methods the flow characteristic of an opening degree control valve for controlling the flow rate of a liquid and a measurement target value are utilized for fuzzy inference to determine an initial valve opening degree prior to the measurement. The fuzzy control of the valve is carried out according to actual measurement values obtained successively to change the valve opening degree.
Furthermore, Japanese Patent Application No. 106413/1987 discloses a closed loop measurement control method in which a measurement target value and fed back actual measurement value are utilized to vary the flow speed. The difference between a measurement target value and an actual measurement value output by a detector adapted to measure a material to be measured and the variation (with time) of the difference are utilized to apply an output to operating means which varies the flow speed by fuzzy control, learning control or optimal control, whereby the flow speed is optimized. This application also discloses an apparatus for practicing the method.
In the closed loop liquid and powder measurement control method in which fuzzy control, learning control or optimal control of flow control means is carried out, a fundamental factor which makes the conventional measuring apparatus disadvantageous (i.e., the condition that the flow speed is constant) is changed. That is, the flow speed is changed by closed loop control. Therefore, a wide range of measurements can be achieved in a short time without being affected by the variation of flow speed due to disturbances and independently of the measurement target values.
However, the method depends greatly on the flow control valve used. That is, if the flow control valve is large in size, then measurement of a small amount of liquid finally remaining takes a relatively long time. On the other hand, if the flow control valve is small in size, then it takes a long time to measure all the liquid.
Heretofore, in order to make measurements more accurate, measuring units have been used which are of the type which invariably limit the flow rate of each liquid in the process of metrically mixing and distributing the liquids.
In the prior art type of liquid measured mixing and distributing apparatus, such measuring units must be respectively associated with supply tanks from which liquids to be metrically mixed are supplied to one tank.
In the case of using a volumetric type of measuring unit as shown in FIG. 19 two measuring units must be provided with two loop control functions for predictive flow rate control corresponding to two types of supplied liquids.
A liquid regulating unit and liquid supply method are disclosed in Japanese Patent Unexamined Publications Nos. 56-74715 (1981) and 58-163426 (1983). According to these publications, the flow rates of liquids are measured by a common measuring unit but liquid supply means for limiting the flow rate are controlled by respectively independent control loops.
This is because, contrary to expectation, highly accurate measurements cannot be attained by one and the same control function since the flow rates of liquids vary according to the quantities of liquids in the supply tanks, flow rate characteristics of valves, physical properties of liquids, and the like.
This applies to the case of measuring units of the tank metering type, in which actuators of stop valves incorporated in respective systems must be controlled by respective independent loop control systems.
To attain a highly accurate measurement, a method has been proposed in which valves having different flow rates are arranged in parallel to each other so as to be switched based on predetermined metering deviation. Also in this method, two loop control functions are required.
The expression "two loop control functions" is used herein for the following reason. In the case of using, for example, a distributed control unit, two control units are not always required because measuring can be made within one and the same control unit. It may however be said that one control unit as viewed in terms of hardware is separated into two control units as viewed in terms of the number of inputs and outputs and software.
Further, the conventional liquid measured mixing apparatus and measuring distribution apparatus are independent of each other regardless of the measuring unit and control unit. Further, the measuring precision in the prior art has been very rough because of using a volumetric flow meter, ON/OFF control of valves, and the like.
The conventional liquid measured mixing apparatus and measuring distribution apparatus have the following disadvantages because the metrical control is made on the assumption that the flow rate is substantially constant.
A first problem in the conventional apparatus is the measuring accuracy.
A change of flow velocity caused by disturbance or caused by a change of liquid physical properties brings about a situation that accuracy cannot be secured.
In the case of gravity transport, the flow velocity of an outflow liquid always changes according to the residual quantity of liquid within each supply tank. The flow velocity may exceed a certain conditional limit if the change of the residual quantity is too large, resulting in deterioration in accuracy.
To improve accuracy, the quantity of liquid within each supply tank must be limited within a certain range to keep the quantity of liquid above a predetermined value, resulting in liquid loss to thereby increase running cost.
A second problem with the conventional apparatus is the measurement range since the measuring range is narrow. This is because, immediately after measuring stops, the inflow of liquid cannot stop because of the delay of response of the system. Because the quantity of the liquid inflow is determined by the flow velocity, the allowable inflow quantity is secured by narrowing the measuring range under the condition that the flow velocity is constant. Accordingly, even in the case where two liquids to be measured are quite the same, measuring units each having a proper measuring range are required, resulting in an increase in number of the units.
A third problem with the conventional apparatus is the measuring time. Since the measuring time is affected by the measurement target value. As the target value becomes smaller, the measuring time becomes shorter, while as the target value becomes larger, the measuring time becomes longer. Accordingly, measuring units suited to the manufacturing cycle are required to correspond to the measured value, resulting in an increase in the number of the units if there are different target values.
Because a large number of independently controlled measuring units are provided for respective supply tanks and distribution tanks and for respective optimum measuring times due to the limitation of manufacturing capacity for the aforementioned reason, the system of the conventional liquid measured mixing and distributing apparatus is complicated. In short, a large number of measuring units are required corresponding to the number of distribution tanks
The present invention has been attained in view of such circumstances, and an object of the present invention is to provide a liquid measured mixing and distributing apparatus in which the distribution system for liquids is constructed as a consecutive system by use of a measuring control unit by which highly accurate metering without influence of a change in flow velocity caused by disturbance or cause by a change in liquid physical properties and short-time metering to secure a wide metering range regardless of the metering set values. Such a system can be attained on the basis of a fuzzy-control liquid measured mixing apparatus described in Japanese Patent Application No. 62 113430 (1987) previously filed by one of the applicants of this application. Particularly a measured mixing apparatus and a measuring distribution apparatus are synthesized as one apparatus to thus attain an increase of manufacturing capacity and a reduction of raw-material loss to produce the following economic effects:
(1) A reduction of initial cost due to the reduction of the number of units;
(2) A reduction of the frequency of maintenance due to the reduction of the number of units;
(3) A reduction of breakdowns due to an improved reliability due to the reduction in the number of units; and
(4) A reduction of running cost due to the reduction of raw-material loss, and the like.
In measuring devices applicable to a conventional liquid/powder measuring-mixing-dispensing system, the flow velocity has been set as a constant and therefore accurate measurement has hardly been attained thereby. In other words, the measuring device has been designed to measure the flow velocity restricted to what corresponds to a set measuring flow instead.
In the liquid/powder measuring-mixing-dispensing system of a conventional type designed to mix the liquids or powders supplied from a plurality of supply containers to one mixing container and to dispense the mixed liquid to dispensing containers, each of the supply containers and dispensing containers is equipped with a measuring device attached thereto.
When volumetric metering devices are employed in a liquid/powder measuring-mixing system, for instance, two measuring devices are, as shown in FIG. 20 used respectively for two supply containers of a liquid A and a powder B. Consequently, a control unit will be required to have a double loop function in order to control the quantity of a liquid or powder fluid preliminarily flowing into a mixing container.
In other words, because the speed of the flowing liquid or powder varies with the liquid or powder quantity in the supply container of the liquid A or the powder B, the characteristics of the flow rate regulator and the physical properties of the liquid or powder, no accurate measurement can be expected from a single loop control function.
This is also the case with a tank metering method in which a shut-off valve of an actuator attached to each system has to be controlled by an independently looped control system.
There is a method of installing parallel flow rate regulators with different flow velocities, the regulators being switched with a predetermined measuring variation to implement accurate measurements. Even in this case, however, the double loop control function is required.
The reason for the use of an expression of the double loop control function above is that, though the data can be processed in one control unit, provided a switch unit or the like is employed, for instance, the necessity of more than one physical control unit can be avoided. Notwithstanding, the system still relies on two effective control units in view of the number of input/output terminals and software.
In the batch production process in which a number of liquids or powders are used, the physical properties of these liquids or powders are different and this makes it often impossible to cumulatively measure then in one and the same container. Accordingly, a production system shown in FIG. 21 becomes justified. This production system is arranged so that a plurality of mixing containers (measuring hoppers and measuring-metering tanks) are installed. Mixable liquids and powders are fed into and measured in the same mixing container (measuring tank or hopper as a first mixture), whereas liquids and powders unmixable with the first mixture are accommodated in a separate container hopper or tank). Another mixing container (control tank) for controlling the reaction and preparation is required on the downstream side and consequently the system tends to become complicated.
In a production system where a mixing container (control tank) for controlling the reaction and preparation is fixed, a high initial investment is required in facilities corresponding to the contents of the products when many types are produced. Furthermore, to implement accurate measuring, there are required a number of measuring tanks and control tanks with piping, measuring devices, control units and associated valves which are attached thereto. In this case, the facilities are usable for some types of products but unusable for others and therefore the system becomes highly wasteful of facilities and also causes the initial facility cost to increase. The introduction of a multipurpose production system is being called for but such a production system tends to become further complicated, because not only the piping system but also the attachment devices will have to be altered if it is of a fixed type (e.g., Japanese Patent Application (OPI) Nos. 74715/81, 155412/81, 72015/82 and 81559/79).
As a result, there has recently been proposed a moving batch production system in which a mixing container (metering tanks, control tanks) is movable. When the conventional metering devices are applied to that system, however, the measuring time varies with the size of a measurement target value and, if the target value is large, it takes much time for the measurement and imposes a restriction on the time required to convey the containers in the moving production system. For this reason, the required number of measuring devices is installed in the conventional production system so as not to restrict the time of conveyance. However, this arrangement is likely to conflict with the intended advantages of the moving production system. Further, the length of stay at a station tends to become prolonged in such a system. A large number of measuring devices are required because of the range of measurement target values, restrictions on the measuring time, conditions of measuring precision, etc. Consequently, the time required for the operation of the coupling pipes increases.
As photosensitive materials are dealt with in the process of producing photographic materials, light must be shielded out and the increased number of joints results in the complication of the system, whereas the performance of the products is readily affected by a change of the conveyance cycle.
Moreover, the measuring-dispenser of the mixed liquid does not share the measurement control system with the measuring mixer. Besides, the measuring device attached to each dispensing container is employed to implement a simple open loop measuring control method by means of a level gauge or time metering.
As measurement control presupposing a constant supply flow velocity is performed in the conventional liquid/powder measuring-mixing-dispensing system, the following drawbacks are similar to those of the liquid measuring-mixing system.
(1) Measurement precision: Measurement precision is not always ensured because of the fluctuation of flow velocity resulting from disturbance and changes in the physical properties of the liquids or powders.
More specifically, the selection of a conveyor depends on the physical properties of the powder, e.g., a damper is used to convey granular powder because it offers excellent fluidity, whereas a screw feeder is used to convey powder whose fluidity is poor. However, the flow of the powder cannot be precisely determined but may be changed by the bulkiness or profile of the powder, and disturbances such as vibration. When liquids or powders are gravity-conveyed, the velocity of the flowing-out liquids or powders varies with the residual amounts of the liquids or powders in the supply container, for instance. However, if the residual amount greatly varies, the measuring accuracy will deteriorate as the flow velocity exceeds the set range of conditions.
This means that the quantity of the liquids or powders in the supply container has to be limited to within a certain range so that more than a fixed quantity of the liquids or powders is secured in the container. Otherwise the lose resulting from the residual liquids or powders in the supply container would increase the running cost.
(2) Measuring range is narrow.
The reason for this is that there remains an inflow due to delay in the response of the system to the suspension of measuring. In view of this fact, the allowable inflow is ensured by narrowing the measuring range on the condition that the flow velocity is constant because the inflow is determined by the supply flow velocity. Measuring devices respectively having adequate measuring ranges are required when the set measurement target values greatly differ from each other even if the same liquid or powder is to be measured. Consequently, the number of measuring devices to be installed increases.
(3) Measuring time: The measuring time is influenced by the set measurement target value. The greater the set target value, the shorter the measuring time becomes, and vice versa. Accordingly, the measuring device has to offer proper measurement times in terms of the production cycle in proportion to the set target value. The number of measuring devices increases also in this case.
For the above-described reasons, a number of individually controlled measuring devices are provided for the supply containers respectively at optimum measurement intervals under the restrictions of the production capacity in the conventional liquid/powder measuring-mixing-dispensing system. Accordingly, the system has become complicated in construction, whereas a large number of measuring devices are required to be incorporated in the production facilities. As a dispenser, the system is incapable of high measuring precision, wasteful of liquid and also needs prolonged time for the measurement.